1. Field of the Invention
This invention relates to a device for preforming one or more elementary wires forming a metal reinforcing cord. This cord is especially suitable for reinforcing composite elastomeric matrix products, such as tyres.
In particular, the preforming device according to the present invention is suitable for operating on high carbon content metal wires, which are preferred for manufacturing high elongation cords.
2. Description of the Related Art
The expression “high elongation” is used to indicate the capacity of the reinforcing elements to be stretched under stress, at least initially, to a considerable extent, thanks to the employment of specific materials and/or certain specifically selected geometrical shapes so as to fulfil particular manufacturing phases of tyres and/or conditions of use of tyres.
In particular, these cords, defined as “HE” (High Elongation), present an ultimate elongation between 4% and 10%.
The wires led out of this preforming device according to the invention are subsequently fed to a traditional stranding station known from the art where the wires thus preformed are twisted around the longitudinal axis of the cord thus obtained.
A further object of the present invention is a procedure for manufacturing said cord, comprising the following phases: preforming one or more elementary wires forming said cord by subjecting them to a permanent deformation along their longitudinal development; stranding the elementary wires by means of a helicoidal twisting around the longitudinal axis of the cord.
Furthermore, the present invention relates to a metal cord, preferably a reinforcing cord, obtained by means of a preforming process and of a subsequent stranding of the aforesaid type.
The cord hereof is specifically designed to be used in manufacturing tyre components for motor vehicles but can be easily employed to manufacture other items, such as for example pipes for high pressure fluids, belts, belt conveyors or any other product made of elastomer-based composite material.
As is known, the metal cords usually employed to reinforce elastomeric products are generally made of several elementary wires helicoidally twisted around an axis which coincides with the longitudinal development of the cords themselves.
Preferably said cords are produced by means of stranding machines comprising: a supporting structure; a rotor coupled to said supporting structure which is rotatable according to a predefined axis; a cradle fastened to the supporting structure according to an oscillation axis which coincides with the axis of rotation of the rotor; feeding devices operatively assembled on said cradle and/or on its outside, suitable for feeding one or more elementary wires coming from respective feeding spools, said one or more elementary wires being driven along suitable stranding paths; and preferably at least one preforming device operating on one or more elementary wires in a section of the wires which preceedes the subsequent stranding phase.
This preforming device imposes to said one or more elementary wires a permanent flexure deformation suitable for supporting and improving the subsequent arrangement of the wires according to a helicoidal development which ensures the necessary keeping of the structural compactness of the cord.
Furthermore, it is important to note that these cords, especially when employed in the manufacturing of tyres, are generally required to be provided with high mechanical resistance and to allow a good physico-chemical adhesion with the elastomeric material in which they are embedded, as well as an efficient penetration of said material in the space surrounding each wire of said cord.
In fact it is known that in order to eliminate the risk of the cords undergoing undesired corrosion phenomena once introduced in a tyre, or inside any product made of elastomeric material, it is very important that the elementary wires forming the cords are entirely coated, for their entire superficial extension, by the elastomeric material in which the cord is embedded.
This result, which is more difficult to be achieved when more complex cords are considered, is not easily achieved even when dealing with cords formed by a low number of elementary wires.
In fact, in order to confer the required geometric and structural stability to the cord, the elementary wires forming the cord are compacted, i.e. positioned intimately in contact with one another, leading to the formation of one or more closed cavities inside said cord which extend along the longitudinal development of the cord.
These cavities are closed and, consequently, cannot be reached by the elastomeric material during the normal rubberizing phases of the cord and, as a consequence, corrosion may develop inside said closed cavities and propagate along the elementary wires forming the cord.
As a consequence, this means, for example, that owing to cuts or punctures in the tyre structure, or to any other reason, humidity and/or external agents can penetrate into said closed cavities inevitably starting a rapid process of corrosion of the elementary wires, thus severely compromising the structural resistance of the cord and of the tyre.
Furthermore, the presence of said closed cavities which cannot be reached by the elastomeric material involves a reduced adhesion of the wires to the elastomer, which—above all if said cords are used for manufacturing tyres—in use can cause an undesired tendency of the wires to separate from the elastomer.
An additional disadvantage due to insufficient rubberizing of the wires, caused by the presence of said closed cavities, is the development of fretting of the wires in contact with one another. This generates an inevitable degeneration of resistance to fatigue of the wires and, consequently, of the cord.
An attempt to overcome this type of problem known in the art consists of using so-called “open” cords, where the wires (generally from three to five) are kept distant from one another during the entire rubberizing phase, carried out according to known procedures consisting of keeping a traction load not exceeding five kilograms applied to the cord.
Said cords are, for example, described in U.S. Pat. No. 4,258,543 in the name of the Applicant. These cords allow a greater penetration of the rubber between the wires forming the cords.
However, the cords thus obtained present several problems, especially in use, since the wires forming the cords tend to be distanced also when they are subjected to considerable traction stress during tyre manufacturing and in tyre use. This fact causes undesired geometric and structural instability of the cords which damages the performance of the tyre.
According to a further embodiment of the prior art, so-called double-diameter cords are used, i.e. cords with two pairs of wires where the diameter of the wires of the first pair is suitably differentiated from that of the second pair.
It is also known (see EP Patent 168,857) to make a metal cord having a first pair of elementary wires of equal diameter and a second pair of elementary wires with a diameter smaller than that of the first pair. Said first and second pairs are fed into a conventional internal collection stranding machine after crossing a circular preforming head where the wires of the first and second pair follow paths which ensure differentiated preforming actions with respect to each other.
The cord thus obtained, consequently, presents the pair of wires with a larger diameter helicoidally twisted together and in reciprocal contact, while each wire of the second pair is interposed between the two wires of the first pair and extends in parallel to the latter, being suitably distanced from them.
In this way, the aforesaid closed cavities are eliminated from the cross section of the cord, ensuring total coverage of elementary wires by the elastomeric material used during the rubberizing phase.
However, the suggested technical solution involves that the wires with the smallest diameter are distanced from those with the largest diameter also when the cord is subjected to traction stress in use. This fact, as for the aforesaid “open” cords, causes a certain geometric and structural instability of the cord which is not advantageous.
Furthermore, it is very difficult to confer to the cord thus obtained an accurate and regular geometrical configuration in each point of its longitudinal development since the constant reciprocal position of the wires in the cord is ensured by the particular type of used preforming device but the distance between the wires with the smallest diameter and the wires with the largest diameter tends to vary randomly in the various points of the longitudinal development, both in conditions of rest and of use of the cord.
According to a further preforming method for known is in the art and described in the aforesaid U.S. Pat. No. 4,258,543 in the name of the Applicant, a roller preforming machine can be used. The roller is idle and presents several preforming seats, each located so as to operatively engage a respective elementary wire of the cord.
These preforming seats are circumferential grooves in the surface of the roller, the width of which is substantially equal to the diameter of the corresponding elementary wire, with a semicircular profile end portion having an axis coplanar to that of the end portions of the other circumferential grooves.
In this way, preforming can be varied by adjusting the radius of curvature of said grooves or by adjusting the tension applied to the wire. However, even this solution presents problems since the preforming action operated on the wire is often thwarted by the dynamic stranding pulls.
To solve the problem of poor rubberizing of the wires of a given cord—fact which can, as mentioned, cause consequent undesired corrosion problems—a suggested solution consists of cords generally formed with a low number of wires, where at least one of the elementary wires is deformed during preforming so as to acquire a pattern which is no longer continuous but presents a suitable broken line.
Such embodiment is described, for example, in U.S. Pat. No. 5,020,312 according to which at least one wire of a given cord is subjected to a zigzag pattern along the longitudinal direction of said cord.
This renders a continuous contact between at least two adjacent wires along the longitudinal development of the cord impossible, thus causing the formation of detachment areas between said two wires, i.e. inlet openings allowing the introduction of rubberizing material at each zigzag bend of the wire.
According to the matter disclosed in this document, one or more wires suitable for forming a given cord are unwound from respective storage spools and fed to a pair of opposed cog wheels through which the above mentioned one or more wires are passed and preformed according to the axial direction conferring the aforesaid zigzag pattern.
This type of preforming is exhaustively described and illustrated in detail also in U.S. Pat. No. 5,581,990.
However, the greatest problem presented by the cords manufactured according to this operative method resides in a remarkable crushing of the external fibers of the wires forming a given cord at the bending apex. This fact involves an inevitable and undesired decrease in the fatigue resistance values of said cord and, consequently, a decrease in the qualitative level of the tyre in which said method is used.
Furthermore, it is known to use preforming devices provided with preforming heads for impressing an axial deformation to one or more of said wires. More in particular, U.S. Pat. No. 5,319,915 discloses the positioning of a flat surface, which extends in parallel to the axis of a wire, before stranding. Said flat surface is provided with preforming heads consisting of several pins positioned perpendicularly with respect to this flat surface at a regular distance from one another.
As illustrated in U.S. Pat. No. 5,722,226, said pins can be located on a supporting structure which may also be conical or cylindrical (i.e. not necessarily flat) and may be aligned or suitably staggered to provide the wire to be preformed with the desired zigzag path.
This device, consequently, is positioned so that said wire passes alternatively over and under said sequence of heads, while the entire device is rotated around its axis which is parallel to the axis of the wire.